gecko-dev/toolkit/components/promiseworker/PromiseWorker.jsm

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

/**
 * A wrapper around ChromeWorker with extended capabilities designed
 * to simplify main thread-to-worker thread asynchronous function calls.
 *
 * This wrapper:
 * - groups requests and responses as a method `post` that returns a `Promise`;
 * - ensures that exceptions thrown on the worker thread are correctly deserialized;
 * - provides some utilities for benchmarking various operations.
 *
 * Generally, you should use PromiseWorker.jsm along with its worker-side
 * counterpart PromiseWorker.js.
 */

"use strict";

var EXPORTED_SYMBOLS = ["BasePromiseWorker"];

ChromeUtils.defineModuleGetter(
  this,
  "PromiseUtils",
  "resource://gre/modules/PromiseUtils.jsm"
);

/**
 * An implementation of queues (FIFO).
 *
 * The current implementation uses one array, runs in O(n ^ 2), and is optimized
 * for the case in which queues are generally short.
 */
function Queue() {
  this._array = [];
}
Queue.prototype = {
  pop: function pop() {
    return this._array.shift();
  },
  push: function push(x) {
    return this._array.push(x);
  },
  isEmpty: function isEmpty() {
    return !this._array.length;
  },
};

/**
 * Constructors for decoding standard exceptions received from the
 * worker.
 */
const EXCEPTION_CONSTRUCTORS = {
  EvalError(error) {
    let result = new EvalError(error.message, error.fileName, error.lineNumber);
    result.stack = error.stack;
    return result;
  },
  InternalError(error) {
    let result = new InternalError(
      error.message,
      error.fileName,
      error.lineNumber
    );
    result.stack = error.stack;
    return result;
  },
  RangeError(error) {
    let result = new RangeError(
      error.message,
      error.fileName,
      error.lineNumber
    );
    result.stack = error.stack;
    return result;
  },
  ReferenceError(error) {
    let result = new ReferenceError(
      error.message,
      error.fileName,
      error.lineNumber
    );
    result.stack = error.stack;
    return result;
  },
  SyntaxError(error) {
    let result = new SyntaxError(
      error.message,
      error.fileName,
      error.lineNumber
    );
    result.stack = error.stack;
    return result;
  },
  TypeError(error) {
    let result = new TypeError(error.message, error.fileName, error.lineNumber);
    result.stack = error.stack;
    return result;
  },
  URIError(error) {
    let result = new URIError(error.message, error.fileName, error.lineNumber);
    result.stack = error.stack;
    return result;
  },
};

/**
 * An object responsible for dispatching messages to a chrome worker
 * and routing the responses.
 *
 * Instances of this constructor who need logging may provide a method
 * `log: function(...args) { ... }` in charge of printing out (or
 * discarding) logs.
 *
 * Instances of this constructor may add exception handlers to
 * `this.ExceptionHandlers`, if they need to handle custom exceptions.
 *
 * @param {string} url The url containing the source code for this worker,
 * as in constructor ChromeWorker.
 *
 * @constructor
 */
var BasePromiseWorker = function(url) {
  if (typeof url != "string") {
    throw new TypeError("Expecting a string");
  }
  this._url = url;

  /**
   * A set of methods, with the following
   *
   * ConstructorName: function({message, fileName, lineNumber}) {
   *   // Construct a new instance of ConstructorName based on
   *   // `message`, `fileName`, `lineNumber`
   * }
   *
   * By default, this covers EvalError, InternalError, RangeError,
   * ReferenceError, SyntaxError, TypeError, URIError.
   */
  this.ExceptionHandlers = Object.create(EXCEPTION_CONSTRUCTORS);

  /**
   * The queue of deferred, waiting for the completion of their
   * respective job by the worker.
   *
   * Each item in the list may contain an additional field |closure|,
   * used to store strong references to value that must not be
   * garbage-collected before the reply has been received (e.g.
   * arrays).
   *
   * @type {Queue<{deferred:deferred, closure:*=}>}
   */
  this._queue = new Queue();

  /**
   * The number of the current message.
   *
   * Used for debugging purposes.
   */
  this._id = 0;

  /**
   * The instant at which the worker was launched.
   */
  this.launchTimeStamp = null;

  /**
   * Timestamps provided by the worker for statistics purposes.
   */
  this.workerTimeStamps = null;
};
BasePromiseWorker.prototype = {
  log() {
    // By Default, ignore all logs.
  },

  /**
   * Instantiate the worker lazily.
   */
  get _worker() {
    if (this.__worker) {
      return this.__worker;
    }

    let worker = (this.__worker = new ChromeWorker(this._url));

    // We assume that we call to _worker for the purpose of calling
    // postMessage().
    this.launchTimeStamp = Date.now();

    /**
     * Receive errors that have been serialized by the built-in mechanism
     * of DOM/Chrome Workers.
     *
     * PromiseWorker.js  knows how  to  serialize a  number of  errors
     * without    losing   information.    These   are    treated   by
     * |worker.onmessage|.   However, for  other  errors,  we rely  on
     * DOM's mechanism  for serializing errors, which  transmits these
     * errors through |worker.onerror|.
     *
     * @param {Error} error Some JS error.
     */
    worker.onerror = error => {
      this.log(
        "Received uncaught error from worker",
        error.message,
        error.filename,
        error.lineno
      );
      error.preventDefault();
      let { deferred } = this._queue.pop();
      deferred.reject(error);
    };

    /**
     * Receive messages from the worker, propagate them to the listeners.
     *
     * Messages must have one of the following shapes:
     * - {ok: some_value} in case of success
     * - {fail: some_error} in case of error, where
     *    some_error is an instance of |PromiseWorker.WorkerError|
     *
     * Messages may also contain a field |id| to help
     * with debugging.
     *
     * Messages may also optionally contain a field |durationMs|, holding
     * the duration of the function call in milliseconds.
     *
     * @param {*} msg The message received from the worker.
     */
    worker.onmessage = msg => {
      this.log("Received message from worker", msg.data);
      let handler = this._queue.pop();
      let deferred = handler.deferred;
      let data = msg.data;
      if (data.id != handler.id) {
        throw new Error(
          "Internal error: expecting msg " +
            handler.id +
            ", " +
            " got " +
            data.id +
            ": " +
            JSON.stringify(msg.data)
        );
      }
      if ("timeStamps" in data) {
        this.workerTimeStamps = data.timeStamps;
      }
      if ("ok" in data) {
        // Pass the data to the listeners.
        deferred.resolve(data);
      } else if ("fail" in data) {
        // We have received an error that was serialized by the
        // worker.
        deferred.reject(new WorkerError(data.fail));
      }
    };
    return worker;
  },

  /**
   * Post a message to a worker.
   *
   * @param {string} fun The name of the function to call.
   * @param {Array} args The arguments to pass to `fun`. If any
   * of the arguments is a Promise, it is resolved before posting the
   * message. If any of the arguments needs to be transfered instead
   * of copied, this may be specified by making the argument an instance
   * of `BasePromiseWorker.Meta` or by using the `transfers` argument.
   * By convention, the last argument may be an object `options`
   * with some of the following fields:
   * - {number|null} outExecutionDuration A parameter to be filled with the
   *   duration of the off main thread execution for this call.
   * @param {*=} closure An object holding references that should not be
   * garbage-collected before the message treatment is complete.
   * @param {Array=} transfers An array of objects that should be transfered
   * to the worker instead of being copied. If any of the objects is a Promise,
   * it is resolved before posting the message.
   *
   * @return {promise}
   */
  post(fun, args, closure, transfers) {
    return async function postMessage() {
      // Normalize in case any of the arguments is a promise
      if (args) {
        args = await Promise.resolve(Promise.all(args));
      }
      if (transfers) {
        transfers = await Promise.resolve(Promise.all(transfers));
      } else {
        transfers = [];
      }

      if (args) {
        // Extract `Meta` data
        args = args.map(arg => {
          if (arg instanceof BasePromiseWorker.Meta) {
            if (arg.meta && "transfers" in arg.meta) {
              transfers.push(...arg.meta.transfers);
            }
            return arg.data;
          }
          return arg;
        });
      }

      let id = ++this._id;
      let message = { fun, args, id };
      this.log("Posting message", message);
      try {
        this._worker.postMessage(message, ...[transfers]);
      } catch (ex) {
        if (typeof ex == "number") {
          this.log("Could not post message", message, "due to xpcom error", ex);
          // handle raw xpcom errors (see eg bug 961317)
          throw new Components.Exception("Error in postMessage", ex);
        }

        this.log("Could not post message", message, "due to error", ex);
        throw ex;
      }

      let deferred = PromiseUtils.defer();
      this._queue.push({ deferred, closure, id });
      this.log("Message posted");

      let reply;
      try {
        this.log("Expecting reply");
        reply = await deferred.promise;
      } catch (error) {
        this.log("Got error", error);
        reply = error;

        if (error instanceof WorkerError) {
          // We know how to deserialize most well-known errors
          throw this.ExceptionHandlers[error.data.exn](error.data);
        }

        if (error instanceof ErrorEvent) {
          // Other errors get propagated as instances of ErrorEvent
          this.log(
            "Error serialized by DOM",
            error.message,
            error.filename,
            error.lineno
          );
          throw new Error(error.message, error.filename, error.lineno);
        }

        // We don't know about this kind of error
        throw error;
      }

      // By convention, the last argument may be an object `options`.
      let options = null;
      if (args) {
        options = args[args.length - 1];
      }

      // Check for duration and return result.
      if (
        !options ||
        typeof options !== "object" ||
        !("outExecutionDuration" in options)
      ) {
        return reply.ok;
      }
      // If reply.durationMs is not present, just return the result,
      // without updating durations (there was an error in the method
      // dispatch).
      if (!("durationMs" in reply)) {
        return reply.ok;
      }
      // Bug 874425 demonstrates that two successive calls to Date.now()
      // can actually produce an interval with negative duration.
      // We assume that this is due to an operation that is so short
      // that Date.now() is not monotonic, so we round this up to 0.
      let durationMs = Math.max(0, reply.durationMs);
      // Accumulate (or initialize) outExecutionDuration
      if (typeof options.outExecutionDuration == "number") {
        options.outExecutionDuration += durationMs;
      } else {
        options.outExecutionDuration = durationMs;
      }
      return reply.ok;
    }.bind(this)();
  },

  /**
   * Terminate the worker, if it has been created at all, and set things up to
   * be instantiated lazily again on the next `post()`.
   * If there are pending Promises in the queue, we'll reject them and clear it.
   */
  terminate() {
    if (!this.__worker) {
      return;
    }

    try {
      this.__worker.terminate();
      delete this.__worker;
    } catch (ex) {
      // Ignore exceptions, only log them.
      this.log("Error whilst terminating ChromeWorker: " + ex.message);
    }

    let error;
    while (!this._queue.isEmpty()) {
      if (!error) {
        // We create this lazily, because error objects are not cheap.
        error = new Error("Internal error: worker terminated");
      }
      let { deferred } = this._queue.pop();
      deferred.reject(error);
    }
  },
};

/**
 * An error that has been serialized by the worker.
 *
 * @constructor
 */
function WorkerError(data) {
  this.data = data;
}

/**
 * A constructor used to send data to the worker thread while
 * with special treatment (e.g. transmitting data instead of
 * copying it).
 *
 * @param {object=} data The data to send to the caller thread.
 * @param {object=} meta Additional instructions, as an object
 * that may contain the following fields:
 * - {Array} transfers An array of objects that should be transferred
 *   instead of being copied.
 *
 * @constructor
 */
BasePromiseWorker.Meta = function(data, meta) {
  this.data = data;
  this.meta = meta;
};